(a) Field of the Invention
This invention relates in general to the art of crystallization and in particular to a novel process for the preparation of fine crystals. More specifically, this invention relates to a crystallization process specifically adapted to the preparation of fine crystals which are characterized by improved morphological homogeneity and a narrow size distribution.
(b) Description of the Prior Art
Processes of crystallization are extensively utilized in many different branches of industry. In some instances, the processes are characterized by the chemical reaction of two or more compounds to form a reaction product which precipitates from the reaction medium in crystalline form. In other instances the processes are purification procedures in which a solid product to be purified is dissolved in a liquid which is a better solvent for the impurities to be removed than for the product itself and the product is precipitated in crystalline form while the impurities remain in solution. In purification processes, various means can be utilized to initiate the crystallization, for example, alteration of the temperature or addition of a second liquid to the system in order to change the solubility limit and thereby cause precipitation to take place. Both in the instance of a chemical reaction forming a crystalline product and the use of crystallization as a purification technique, the characteristics of the resulting crystals are significantly influenced by such factors as the operating conditions of the process and the type of equipment used in carrying out the process.
Among the important properties of a crystalline product are the morphology of the crystals, their dimensions, and their size distribution. In some instances it is highly desirable that a crystalline product be produced in a very finely-divided state, i.e., in the form of fine crystals such as crystals having a particle size of about one micron or less. Examples of products in which this can be an important criterion are metal oxalates which are to be used in the production of magnetic particles intended for use in magnetic recording, phosphorescent compounds which are used, for example, in the preparation of X-ray intensifying screens, and silver halides used in the manufacture of radiation-sensitive photographic materials. Many other examples of products in which a fine crystalline form is advantageous can also be mentioned, such as pigments for use in paints, catalysts used in heterogeneous catalysis, and pharmaceutical products whose physiological effectiveness is influenced by their surface area. In addition to producing crystals of fine size, it is often important to be able to produce crystals which are morphologically homogeneous, i.e., which do not differ significantly from one to another in crystalline form, and which are all of substantially the same size, i.e., within a narrow size distribution.
The prior art has encountered difficulty in producing fine crystals characterized by morphological homogeneity and a narrow size distribution, particularly in a continuous or flow through preparation mode. Thus, for example, differences in temperature or in concentration at different points within the vessel in which crystallization is carried out, especially when the vessel is of large size, contribute toward the formation of a crystalline product with a broad size distribution. In many crystallization processes there is a tendency for a crust to form on the walls of the vessel, especially when the walls are subjected to cooling, and this crust can be composed of interlaced crystals forming a separate population from the main body of crystals, i.e., displaying different morphological characteristic. Back-mixing within the reaction vessel can lead to very disparate crystal sizes. Also, random nucleation due to poor control of the conditions at the beginning of the crystallization can lead to crystals of disparate morphology and dimensions. Reproducibility of the process may be especially unsatisfactory. Attempts to avoid these problems have been subject to extensive effort in the past by persons working in the field of crystallization processes. Many different approaches seeking to solve one or more of the aforesaid problems have been described in the patent literature, for example, in U.S. Pat. No. 3,190,748 pertaining to the manufacture of metal oxalates, in British Pat. No. 1,365,536 pertaining to the use of crystallization columns in purification and/or concentration processes, in U.S. Pat. No. 3,668,142 pertaining to the preparation of phosphors, and in U.S. Pat. Nos. 3,801,326 and 3,897,935 pertaining to the manufacture of silver halide crystals.
It is toward the objective of providing a simple, practical and highly efficient process for preparation of fine crystals characterized by morphological homogeneity and a narrow size distribution that the present invention is directed.